This invention relates to wastewater treatment processes, and more particularly, to such processes involving the precipitation of metal ions as sulfides.
Much importance has recently been placed on the reduction of effluent levels of priority pollutants from industrial process waste streams due to environmental considerations. These pollutants include heavy metals such as found in wastewater resulting from electroplating operations. A conventional process for removing heavy metals from a wastewater stream is based on the precipitation of metal hydroxides through the addition of a base such as lime or caustic soda to the stream. Resulting metal hydroxide precipitates are then removed by flocculation, clarification and often by filtration. Although effective, this process has limited applicability due to the high solubilities and amphoteric properties associated with metal hydroxides. This process is also not very effective in the presence of chelating agents commonly used in metal finishing operations. Similarly, it is not effective for removing certain metals such as cadmium, silver and lead.
Sulfide precipitation is an alternative wastewater treatment process which advantageously exploits the extremely low solubilities characteristic of metal sulfides. Additionally, metal sulfides are not amphoteric. Accordingly, heavy metals can be removed to extremely low concentrations with the high stability of metal sulfides enabling the precipitation of metals even in the presence of chelating agents. In addition, resulting metal sulfide sludges have better dewatering characteristics than comparable metal hydroxide sludges. Although sulfide precipitation has such advantages, it also has several disadvantages including toxicity, the odor of H.sub.2 S evolved in the process, and pollution of treated wastewater with excess sulfide ions. These problems typically arise when sulfide ion addition to a wastewater stream is not carefully controlled. Of course, the addition of sulfide sources in excess of that required also degrades the economics of a wastewater treatment process.
There are two principal sulfide precipitation processes currently available. These processes differ in the technique of delivering metal sulfide ions to an aqueous solution and are termed soluble sulfide and insoluble sulfide processes, respectively. In the soluble sulfide process Na.sub.2 S or NaHS solutions are typically added to a wastewater stream. The addition of the solution may be controlled through reference to periodic analysis of wastewater metal content, or alternatively, through the use of a feedback control system employing an ion specific electrode. In either case, the addition of soluble sulfides usually produces colloidal or very fine particles with poor settling characteristics. Accordingly, the particulates are often treated with coagulants and flocculants before final charification of the wastewater stream.
The insoluble sulfide method uses a sparingly soluble metal sulfide as a source of sulfide ions. For example, the process described in U.S. Pat. No. 4,102,784 (Schlauch) uses freshly prepared ferrous sulfide. The addition of the ferrous sulfide is adjusted in response to the concentration of pollutant ions in a wastewater stream as determined by standard jar tests on the wastewater before entering a sulfide precipitation tank. Since ferrous sulfide has a very low solubility the probability of H.sub.2 S emission or wastewater contamination by excess sulfide ions is reduced as compared to the soluble sulfide process. However, the low solubility of ferrous sulfide necessitates the addition of 2 to 4 times the stoichiometric amount. Of course, the use of excess ferrous sulfide adds to the chemicals cost of the process. More importantly, large amounts of waste sludge result from the addition of excess ferrous sulfide. For example, the Sulfex.RTM. process which employs ferrous sulfide can produce as much as three times the sludge produced in conventional hydroxide precipitation processes.
Indeed, one of the major problems historically encountered in chemical precipitation cleanup processes has been the disposal of resulting waste sludges. The Resource Conservation and Recovery Act of 1976 (Public Law 94-580) has imposed regulations covering the disposal of toxic industrial sludges. Due to these regulations, the disposal of sludges containing toxic wastes incur significant cost penalties. Accordingly, whereas it is desirable to reduce the total amount of waste sludge produced in a wastewater treatment process, it is especially desirable to specifically reduce the amount of toxic sludge produced therein.
Thus, it is an object of the present invention to provide an improved wastewater treatment process which enables a substantially complete removal of pollutant metal ions, even in the presence of chelating agents.
It is also an object of the present invention to reduce the total amount of the sludge produced in a wastewater treatment process, and more specifically, to reduce the amount of toxic sludge so produced.
It is a further object of the present invention to enable a simplified continuous control of sulfide addition in a wastewater treatment process.
Still another object of the present invention to provide a simple and economical method for providing a calcium sulfide mixture for use in a sulfide precipitation wastewater treatment process.